Command cursor should use command reported namespace for getMore

This is a regression in PyMongo 3.2. The changes for PYTHON-978 caused CommandCursor to use <database name>.<collection name> as the namespace for getMore operations, regardless of the 'ns' value returned from the initial command.

This may have no practical current effect on applications since in most cases the 'ns' value returned from the initial command will be <database name>.<collection name>.

Discovered by mockupdb tests. See also PYTHON-806.